This invention relates to chromatic dispersion in optical systems, and more particularly to detection or discrimination of chromatic dispersion in optical signals used by optical transmission systems.
Current optical fibers have a property known as chromatic dispersion which causes light transmitted along the fiber to experience an amount of propagation delay that is dependent on the frequency, or wavelength, of the light. Optical signals transmitted over these fibers by optical transmission systems are modulated carrier signals which exhibit an optical bandwidth determined by the upper and lower modulation sidebands containing different optical frequencies (higher for upper sideband, lower for lower sideband). These different frequency components of an optical signal will experience different amounts of propagation delay, depending on the frequency (or wavelength) of the component, as the optical signal travels along the fiber, resulting in non-coincident (in time) reception at the far end receiver. The resulting variance in propagation delays among the different frequency components changes the optical signal, thereby making error-free demodulation of the signal more difficult.
At a particular frequency, an optical fiber has a xe2x80x9cnullxe2x80x9d point at which the propagation speed is highest and hence the propagation delay is minimum. On either side of this peak propagation speed, propagation speed diminishes and propagation delay increases. Since dispersion is defined as the change in propagation delay relative to frequency (or wavelength), the dispersion at the null point will be zero and it will be opposite in polarity on either side of this point. The positive dispersion of one type of fiber can be used to approximately compensate for the negative dispersion of another type of fiber, and in this way optical links can be engineered to have minimal dispersion over a narrow frequency (or wavelength) range.
However, in dense wavelength division multiplexed (DWDM) systems, which typically have anywhere from 40 to 160 DWDM optical signals modulated on carriers spaced apart at 50-100 GHz and using optical carriers in the 1520-1580 nm range, engineering optical links to provide minimal dispersion for all of the DWDM optical signals is difficult, if at all possible, because of the wide range in frequency (or wavelength) of the signals. This becomes more difficult still as new transmission bands are added (L=long wavelength, beyond about 1600 nm, S=short wavelength, below about 1500 nm). Typically, the amount of dispersion imparted on a group of DWDM optical signals transmitted over an optical link will vary significantly across the range of signals. When these DWDM signals are switched with optical signals from other optical links having different dispersion characteristics, the result is a new group of DWDM optical signals having an even wider, and now non-systematic variance in dispersion across the range of signals. This result is most prevalent in versions of automatically switched optical networks (ASON) which use purely photonic switches (as opposed to electro-optical switches and transponders) because the links over which optical signals travel between source and destination nodes in the network change dynamically to adapt to changing traffic demands placed on the network.
Therefore, it would be desirable to have a means of detecting the amount of dispersion in individual optical signals received over a dispersive optical link or at least discriminating which polarity of chromatic dispersion is present, thereby allowing the correct amount of dispersion compensation to be applied to each optical signal, in either an open-loop (magnitude/polarity detection) or closed loop (residual polarity detection) application.
The invention uses the chromatic dispersive properties of two or more different types of optical fibers in order to determine the polarity and magnitude of dispersion that a received optical signal has undergone as a result of being transmitted over one or more dispersive optical links. Embodiments of the invention offer the advantage of allowing dispersion detection or discrimination to be performed on a per wavelength basis which is the first step to enabling compensation to be performed on individual optical signals on the basis of the amount of dispersion each optical carrier signal has undergone during transmission over an optical link, thereby allowing for more accurate dispersion compensation as compared to means employing engineered links which provide predetermined dispersion compensation. Embodiments of the invention also offer the advantage of allowing the dispersion of an optical link to be measured without requiring knowledge of the spectrum of the optical signal travelling on that link.
According to a first broad aspect of the present invention, there is provided a dispersion discriminator for determining the amount of dispersion in an amplitude modulated optical signal. The amplitude modulated optical signal is a double side band signal such as may be produced by Amplitude Shift Keying (ASK), which is the on-off, or quasi on-off, amplitude modulation used in conventional optical systems. The dispersion discriminator includes a first dispersion arm for causing a first additional amount of dispersion in a first portion of the optical signal and a second dispersion arm for causing a second additional amount of dispersion in a second portion of the optical signal, the first and second additional amounts being of opposite polarities. Preferably, the magnitude of the first additional amount of dispersion is substantially equal to the magnitude of the second additional amount of dispersion, which leads to technical advantages such as increased detection sensitivity.
The dispersion discriminator further includes a dispersion detector capable of receiving the first and second portions of the optical signal from the respective dispersion arms; detecting, for each of a plurality of electrical frequencies, a difference in a characteristic of the first and second portions of the optical signal; determining a particular electrical frequency at which said difference falls outside a predetermined range; and mapping the particular electrical frequency and the difference at the particular electrical frequency to a magnitude of dispersion in the optical signal.
According to a second broad aspect, the present invention may be summarized as a method of determining an amount of dispersion in an amplitude modulated optical signal from first and second portions of the optical signal, the first portion of the optical signal having travelled along a first dispersion arm that adds a first additional amount of dispersion, the second portion of the optical signal having travelled along a second dispersion arm that adds a second additional amount of dispersion, the first and second additional amounts being of opposite polarity. The method includes receiving the first and second portions of the optical signal from the respective dispersion arms, detecting (for each of a plurality of electrical frequencies) a difference in a characteristic of the first and second portions of the optical signal, determining a particular electrical frequency at which said difference falls outside a predetermined range and mapping the particular electrical frequency and the difference at the particular electrical frequency to a magnitude of dispersion in the optical signal.
These and various other aspects of the present invention will best be understood upon a reading of the following detailed description of specific embodiments of the invention in conjunction with the accompanying drawings.